# Energy and Respiration (A2 Portion)

> CIE A-Level Biology · CIE 9700 A2
> Source: https://www.owlsprep.com/study/cie-9700-u13-overview/
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This A2 unit deepens your understanding of cellular energy production, covering core respiratory stages after glycolysis, energy yield calculations, alternative pathways, and experimental methods to measure respiration rate.

**Prerequisites:** AS Level knowledge of glycolysis and eukaryotic cell structure

## Learning objectives

- Describe the location, reactions and products of the Krebs cycle in aerobic respiration
- Explain the process of oxidative phosphorylation and chemiosmosis in ATP production
- Compare aerobic and anaerobic respiration pathways across different organisms
- Calculate respiratory quotients and compare energy yield from different respiratory substrates
- Interpret experimental data from investigations measuring respiration rate

## Unit at a Glance

This unit follows on from the AS introduction to respiration, walking you through each stage of aerobic respiration in detail before exploring alternative pathways and practical assessment skills. You will build from understanding where each reaction occurs in the mitochondrion to calculating overall net ATP production from different food sources.

The learning sequence progresses logically: from the cyclical reactions of the Krebs cycle (which produce reduced coenzymes for energy production), through to ATP synthesis, then anaerobic respiration, energy from different substrates, and finally practical measurement of respiration rate.

This unit is split into 5 core sub-topics:
- [Krebs cycle](https://www.owlsprep.com/study/cie-9700-u13-krebs-cycle/) — Detailed walkthrough of the reactions, products, and location of the Krebs cycle in aerobic respiration.
- [Oxidative phosphorylation](https://www.owlsprep.com/study/cie-9700-u13-oxidative-phosphorylation/) — Explains the electron transport chain, chemiosmosis, and how most ATP is produced in aerobic respiration.
- [Anaerobic respiration](https://www.owlsprep.com/study/cie-9700-u13-anaerobic-respiration/) — Compares anaerobic pathways in animals, yeast and bacteria, and their net ATP yield compared to aerobic respiration.
- [Respiratory substrates](https://www.owlsprep.com/study/cie-9700-u13-respiratory-substrates/) — Covers energy release from carbohydrates, lipids and proteins, and how to calculate RQ values.
- [Measurement of respiration rate](https://www.owlsprep.com/study/cie-9700-u13-measurement-of-respiration-rate/) — Explains how to use a respirometer to measure respiration rate and interpret experimental data.

## Common pitfalls

- **Wrong:** Confusing Krebs cycle products per glucose vs per pyruvate
  - Why it fails: One glucose splits into two pyruvate molecules, so all Krebs cycle outputs are doubled for a single glucose molecule
  - Correct: Always multiply product counts by 2 when calculating total yield from one starting glucose molecule
- **Wrong:** Overestimating ATP contribution from the Krebs cycle
  - Why it fails: Most ATP comes from oxidative phosphorylation, not substrate-level phosphorylation in earlier stages
  - Correct: Recall that ~30 of 32 total ATP per glucose comes from oxidative phosphorylation
- **Wrong:** Mixing up RQ values for different respiratory substrates
  - Why it fails: RQ value depends on the oxygen content of the substrate, which varies between macronutrients
  - Correct: Memorize RQ = 1 for carbohydrates, 0.7 for lipids, and 0.9 for proteins

## Cheatsheet

| Concept | Key Fact / Value |
| --- | --- |
| Location of Krebs cycle | Mitochondrial matrix |
| Location of oxidative phosphorylation | Inner mitochondrial membrane |
| Net ATP per aerobic glucose (eukaryotes) | 30-32 ATP |
| Net ATP per anaerobic glucose | 2 ATP |
| Respiratory Quotient formula | RQ = $\frac{CO_2\ produced}{O_2\ consumed}$ |
| RQ for carbohydrates | 1.0 |
| RQ for lipids | 0.7 |
| RQ for proteins | 0.9 |

## What's next

Begin your study of this unit with the first sub-topic, which covers the Krebs cycle, the first core A2 stage of aerobic respiration after glycolysis. Work through each sub-topic in order to build your understanding sequentially, as later concepts depend on knowledge of earlier stages. Once you complete all sub-topics in this unit, you can move on to the next A2 unit to continue your syllabus progress.

- [Krebs cycle (First Sub-Topic)](https://www.owlsprep.com/study/cie-9700-u13-krebs-cycle/)
- [Photosynthesis (Next Unit Overview)](https://www.owlsprep.com/study/cie-9700-u14-overview/)
- [Oxidative phosphorylation](https://www.owlsprep.com/study/cie-9700-u13-oxidative-phosphorylation/)

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